The seafloor is inhabited by a large diversity of organisms and plays an important role in the decomposition of organic material and the recycling of nutrients to the overlying water. Bottom trawling impacts seafloor habitats and communities and disturbs bio-geochemical processes by mixing and mobilisation of the sediment. To support sustainable fisheries management, methods have been developed to quantify the trawling impacts on the community biomass and composition. However, a method to quantify the mobilisation of fine sediment by different bottom trawls is lacking.
The latest Editor's Choice article attempts to fill this gap by adding an urgently needed tool to the box of trawling impact assessment methodologies. In "Sediment mobilization by bottom trawls: a model approach applied to the Dutch North Sea beam trawl fishery", the method developed enables scientists to estimate the amount of sediment mobilised by bottom trawls based on the dimensions of the bottom trawl components and silt content of the fishing grounds. The method builds on knowledge that the amount of sediment mobilised is a function of the hydrodynamic drag of the trawl and the percentage of fine particles of the sediment. By decomposing a bottom trawl into its main components and estimating the hydrodynamic drag and sediment mobilisation of each component from its shape, frontal surface area and speed, the sediment mobilised by the whole gear can be estimated by summing up the sediment mobilisation of the individual components. The method is applied to the Dutch beam trawl fishery for flatfish using either conventional tickler chain beam trawls or electric pulse trawls. A comparison with in situ measurements from the literature showed that our approach provides reasonable estimates of the quantity of sediment mobilized by different gears.
With the development and validation of this component-based method for quantifying drag and sediment mobilisation we have provided policy-makers and fisheries managers with a quantitative means to assess the physical impacts of different fishing gears and fishing methods across sediment types. The method allows ranking of different gears in terms of their impact as well as a direct comparison with the physical impact of natural events (such as storms and tides) and of other uses of the seabed (such as mineral extraction and mining). Accordingly, it will permit a rationale and objective approach to fulfilling the requirements of the Common Fisheries Policy and the Marine Strategy Framework Directive. The method presented by the authors could also be used to provide estimates of trawling-induced sediment mobilisation for mechanistic models of biogeochemical cycle and hence, improve understanding of trawling impacts on these processes. Furthermore, the component-based estimation of hydrodynamic drag will provide a better understanding of the forces required to tow a trawl gear across the seabed and contribute to the development of more fuel-efficient gears that will reduce CO2 and NOx emissions from the fishing industry.
Conventional Sumwing beam trawl with tickler chains (left) and Pulse wing with electrodes (right) used in the beam trawl fishery for sole in the North Sea.